This invention relates to the field of communications, and in particular to the processing of multiple asynchronous spread-spectrum communications.
The Spread-Spectrum techniques are used to modulate an information signal such that the modulated signal appears as noise. The information is modulated by a pseudo-random signal, and can be demodulated, or despread, by the same pseudo-random sequence. This modulation is commonly referred to as Direct-Sequence Spread Spectrum (DSSS). The modulated signal is spread across a bandwidth that is substantially larger than the bandwidth of the information signal, and has the apparent effect of increasing the noise-floor of receivers that receive this signal. Applying the same pseudo-random sequence to the modulated signal allows the information signal to be detected within this apparent noise.
Code Division Multiple Access (CDMA) is a commonly used spread-spectrum communications technique wherein the information signal is encoded by one of many code sequences before it is transmitted. The received signal is decoded by the same code sequence to reproduce the original information signal. Transmissions from multiple transmitters can be simultaneously communicated via a common frequency channel by employing different code sequences for each transmitter, provided that the code sequences have particular uniqueness characteristics. The uniqueness characteristics of acceptable codes substantially guarantee that a coherent output will only be produced when the received signal corresponds to a signal that is encoded using the same code sequence. Signals that are not encoded using the same code sequence as the decoding code sequence are decoded as noise signals. In a conventional CDMA system, such as a cellular telephone network, the network controller allocates and deallocates code sequences on demand, so that each of the transmitters can transmit over the same network without interference from other transmitters.
A significant characteristic of a pseudo-random spread spectrum code is that a coherent output is only produced when the decoding code sequence is applied substantially in phase with the encoding code sequence. Consider, for example, a six-bit code sequence 0-1-1-0-1-0. A one-bit phase shift of this sequence is 1-1-0-1-0-0 (cyclic shift to the left); a two-bit phase shift is 1-0-1-0-0-1; and so on. If the received signal is decoded with a code-phase that corresponds to an encoding phase shifted by two bits, for example, this would be equivalent to receiving a signal having a 1-0-1-0-0-1 encoding sequence and decoding it with a 0-1-1-0-1-0 sequence. If this six-bit code is a proper pseudo-noise code, having the above defined uniqueness characteristics, then the decoding of this signal having a “different” encoding code merely produces a noise output. U.S. Pat. No. 5,537,397, “SPREAD ALOHA CDMA DATA COMMUNICATIONS”, issued Jul. 16, 1996, to Norman Abramson, and incorporated by reference herein, discloses a technique that uses this phase-dependency characteristic to allow multiple transmitters to use the same code concurrently. As in the conventional CDMA system, the network controller provides an allocation to each transmitter, but in the referenced patent, each transmitter is allocated a different time-slot, or code-phase, rather than a different code. The controller instructs each transmitter to advance or delay its transmission, so that its signal is received at the receiver with a code-phase that is sufficiently different from the code-phase of other transmitters. In this manner, although each of the transmitters and the receiver use the same code, each transmitter provides a “different” (phase-shifted) code to the receiver, relative to the particular code-phase of the decoder at the time of decoding. This is the scheme that is conventionally used in most cell phone systems, wherein the base station assigns a unique phase to each transmitter in its transmission range.
The aforementioned prior art technique requires a unique identification of each mobile transmitter, because the communication of each allocated code or code-phase must be directed to the appropriate transmitter. Each mobile transmitter must also be equipped with a receiver, to receive and process the communicated phase allocation. In conventional cell-phone systems, each base station transmits a pilot signal that the mobile systems use to synchronize their code-phase to the base station's phase. Due to propagation delays and other factors, this synchronization is a ‘coarse’ synchronization that allows the base station to locate the transmissions within a relatively narrow timespan of when the “in-phase” transmissions should be received at the base-station. Once the in-phase signal is received at the base station, a phase-locked loop is used to assure that the base station receiver remains in sync with the mobile transmitter, to compensate for any differences between the transmitter's frequency and the receiver's frequency. That is, a separate phase locked loop is required for each currently active transmitter.
U.S. Pat. No. 6,128,469, “SATELLITE COMMUNICATION SYSTEM WITH A SWEEPING HIGH-GAIN ANTENNA”, issued 3 Oct. 2000 to Ray Zenick, John Hanson, Scott McDermott, and Richard Fleeter, and U.S. Pat. No. 6,396,819, “LOW-COST SATELLITE COMMUNICATION SYSTEM”, issued 28 May 2002 to Richard Fleeter, John Hanson, Scott McDermott, and Ray Zenick, and U.S. Pat. No. 6,317,029, “IN-SITU REMOTE SENSING” issued 13 Nov. 2001 to Richard Fleeter, disclose systems and methods that facilitate the reception and processing of messages from a large number of preferably low-cost transmitters, and are each incorporated by reference herein. For example, a large number of IC chip-size transmitters may be released from an aircraft that overflies a hurricane or forest fire. These transmitters may be configured to periodically or randomly transmit their location and the atmospheric conditions at their location, such as temperature, pressure, moisture content, and so on. A receiving system receives and processes the transmissions from these many devices and provides temperature and pressure profiles, changes and trends, predictions, and the like. Such systems require simple, low-cost, and efficient transmitters.
It would be advantageous to provide a receiving system that is configured to distinguish among transmissions from a plurality of transmitters that are each communicating independently at a common frequency and using a common DSSS code sequence. It would also be advantageous to provide a receiving system that distinguishes among these transmissions as the frequency and transmission rates vary among the plurality of transmitters. It would also be advantageous to provide a receiving system that does not require that each transmitter include a receiver for achieving coarse synchronization with the receiver. It would also be advantageous to provide a receiving system that does not require a separate phase-locked loop for each transmitter.
These objects and others are achieved by providing a receiving system that dynamically searches the communications band for transmissions of messages having the same nominal communications parameters, including the use of the same spreading code, but having potentially different specific frequencies and code-phases. The receiver samples the communications band at each code-phase of the spreading code over a span of down-converted transmission frequencies. When a message element is detected at a particular code-phase and frequency, it is forwarded to a demodulator that demodulates the message and sends it to its intended destination. This technique allows each transmitter to be independent of the receiver. In a preferred embodiment of this invention, a Fast M-Sequence Transform (a Walsh-Hadamard Transform) is used to determine the power level at multiple code-phases at a given frequency in parallel, thereby substantially reducing the time required to search for transmissions at each discrete code-phase.
Throughout the drawings, the same reference numerals indicate similar or corresponding features or functions. The drawings are included for illustrative purposes and are not intended to limit the scope of the invention.